SEE THE DIFFERENCE
Higher ISOs will let you use faster shutter speeds, but at a price
ISO800, 1/15 sec
ISO800 used to be considered very ‘fast’, but today’s Nikon D-SLRs produce good quality even at this setting. You will notice more noise, but the shutter speed is 1/15 sec, which is still marginal but more likely to give a sharp picture.
ISO3200, 1/60 sec
Increasing the ISO to 3200 gives much more noise, but it’s still not too obtrusive at normal viewing distances and it allows a shutter speed that should eliminate any camera shake, especially if your lens has VR built in.
ISO12800, 1/250 sec
At this ISO setting, the picture quality is relatively poor, so you probably want to avoid setting the ISO this high if you possibly can – but it does allow a shutter speed of 1/250 sec, which would freeze most moving subjects.